The invention relates to a method and an apparatus for mixing pulverous material with liquid.
Pulverous materials are mixed with liquids in a variety of processes. For example, a coating material used for coating paper or some other web-like material is prepared by mixing coating material components with each other for a desired coating material. Typically, some of the components are in liquid form, such as water or pigment slurry, and some of the components are in pulverous form that are subsequently admixed with liquid components.
Pulverous components are typically admixed or blended in with liquid components in a batch process by bringing a pulverous component and liquid together in a special mixer and by agitating the components with each other until the pulverous component has dispersed and/or dissolved in the liquid. In the preparation process for coating materials, in particular, a problem is that certain pulverous coating material components have a poor miscibility with liquid components. Particularly problematic mixing processes include e.g. dispersion of dry CMC (carboxymethyl cellulose) in pigment slurry and dispersion of retention polymers in water. Another problem in blending based on the conventional use of mixers is that it is highly complicated, and in many cases even impossible, to integrate it with a continuous preparation process.
Attempts have been made to alleviate the problem by longer mixing times. However, longer mixing times decrease the capacity of an entire preparation process and, respectively, increase the size and costs of mixing equipment. For example, the dispersion of CMC in pigment slurry in available mixing technology typically calls for mixing times of at least 45–90 minutes. This results in a significant fall in capacity as compared to having CMC pre-dissolved in water. However, if CMC is dissolved in water, the dry content of a final coating material falls to an essentially undesirable level. This, in turn, rules out the use of pre-dissolved CMC.
As for retention polymers, attempts have been made to mitigate the problem by means of pre-elutriation of polymers. For example, U.S. Pat. No. 5,857,773 discloses a dissolution method and apparatus for polymers. In the cited method, polymer is fed from a supply tank to a premixing tank by way of a dissolution head, wherein the polymer is supplemented relative to its flow direction first with air and then with water, said components being mixed in a mixing chamber included in the dissolution head upstream of the premixing tank. A problem with the cited method and apparatus, when applied in a coating material preparation process, is however that, during a coating process, the air and air bubbles, present in the coating material, develop roughness of paper surface, and even spots completely void of coating material. The problem is pronounced with certain types of coating materials, which take up more air than other coating materials. Such materials include e.g. coating compositions which contain talc. Therefore, a troublefree application of the cited method and apparatus would in several cases require the use of separate air vents for evacuating the coating material of air admixed with the coating material during the pre-dissolving process. One such air vent for expelling the air contained in a coating material is described in Patent publication FI98792.
U.S. Pat. No. 4,688,945 discloses a mixing apparatus, comprising a mixing chamber for mixing dry polymer with water. The apparatus comprises a tapered, downward convergent mixing chamber. Alongside the mixing chamber is a water supply channel, spiralling around the mixing chamber and having a downward converging profile for increasing the feed rate of water. According to the cited publication, polymer is supplied by a screw conveyor and blended in water fed into the mixing chamber for admixing the polymer with water. The mixture of water and polymer is guided from the mixing chamber into an upright mixing pipe, which is rotatable and functions as a mixer shaft for the actual mixing chamber. The mixture is carried along the mixing pipe to mixer paddles, whereby the mixture is delivered into a mixing tank. A problem with the cited apparatus is that the tapered or conical configuration of a mixing chamber may induce in the water or in the mixture of water and polymer supplied into the chamber, a tendency to strive upwards, thus disturbing the function of the apparatus. In addition, the cited publication does not disclose any feed rate for water, which must be sufficiently high for the water to effectively aspirate the polymer powder along for preventing its adherence to the walls of a supply chamber. Furthermore, the water supply channel disclosed in the cited publication is not practical for use with any liquid other than water, since for example pigment slurry, when delivered by way of a supply channel as described in the cited publication, could, as a result of its composition, obstruct, seriously deteriorate or even break the channel.
Patent publications U.S. Pat. No. 5,122,348, U.S. Pat. No. 3,994,480, GB 2,031,748 A and EP 0460804 A1 also disclose methods and equipment for mixing pulverous material with liquid in a conical mixing chamber. The most serious problem in solutions disclosed in the cited references is that they admix at least a certain amount of air with liquid. Thus, such solutions are not suitable for use in applications, wherein the air diffused in liquid causes problems in subsequent use of a composition produced in the mixer. One such application is, for example, a mixing process for a coating material used for coating paper or board. A further problem with the solutions set forth in the cited references is the conical shape of mixing chambers used therein, which may induce in the water or the mixture of water and polymer supplied into the chamber, a tendency to strive upwards, thus disturbing the function of the apparatus.
Patent publication WO 01/70382 discloses a system and method for mixing solid matter and liquid. According to the cited publication, the mixing of solid matter and liquid is performed in a mixing vessel by introducing liquid from top of the vessel thereinside for rotatory motion, such that the liquid produces an eddy inside the vessel. The magnitude of an eddy-creating centrifugal force is controlled in such a way that the air core or eddy is not able to reach the liquid outlet. Solid matter to be admixed with the liquid is supplied onto the liquid surface. According to what is described in the cited publication, the depth of an eddy present in the mixing vessel is controlled in such a way that the eddy, and the air confined thereby, cannot reach the mixing vessel outlet, thus inhibiting the escape of mixing-vessel contained air from the mixing vessel through the outlet. As set forth in the cited publication, the mixing-vessel contained air is discharged through a filter bag provided in the top section of the mixing vessel. However, a problem with the system and method disclosed in publication WO 01/70382 is, for example, that the supply of the solid matter to be mixed is effected by means of a screw conveyor, whereby agglomerations possibly present in the solid matter do not effectively disintegrate during the actual passage. Furthermore, since the mixing process, is performed in a so-called gentle manner according to the cited publication, there is a risk that agglomerations present in the solid matter do not break up during the course of mixing, and that the dispersion of solid matter within the liquid shall not be sufficiently thorough.